The conventional production of memory components in silicon technology depends on complex. circuits made of transistors, capacitors and resistors. There is a series of prior art memory principles both for volatile (for example DRAM—“Dynamic Random Access Memory”) as well as non-volatile (for example so-called. flash memories). The storing of charges in the silicon-based technology however will reach its scaling limits in the foreseeable future. Furthermore the currently-used memory principles generally require expensive high-temperature processes and are less suitable for three-dimensional integration. There is therefore an intensive search underway worldwide for alternative methods and materials for the permanent storage of information.
Organic electronics has proved very promising as an alternative to silicon-based electronics. The benefits are the comparably simple processes such as printing or vaporising and allowing to condense at low temperatures, the opportunity to work on flexible substrates and the large variety of molecular materials.
There are various prior art memory components with one or more active organic layers. Potember et al.: Applied Physics Letters, Vol. 34, 1979, pages 405-407 “Electrical switching and. memory phenomena in Cu-TCNQ thin films” describe a memory element comprising a copper contact, the active organic material Cu-TCNQ and an aluminium top contact. This memory cell can be reversibly switched by deliberately applying an electrical field between a high resistance (2 MOhm) and a low resistance (200 Ohm). The switching behaviour is caused by a charge transfer complex or a volume effect in the Cu-TCNQ. Adversely in this case is the required thickness of the Cu-TCNQ layer is 10 μm*.
Yang et al.: “Applied Physics Letters, Vol. 80, 2002, pages 2997-2999 “Organic electrical bistable devices and rewritable memory cells” describe a memory cell with an organic active material 2-amino-4,5-imidazole dicarbonitrile (AIDCN). The memory cell comprises several organic layers made of (AIDCN) which enclose a thin aluminium layer. For the switching process this system requires a thin aluminium layer which is embedded between the organic layers and aluminium as the electrode material. A disadvantage of this structure is the need to use aluminium electrodes and the embedded thin aluminium layer, which makes the manufacture more expensive and particularly the ultimately un-clarified switch mechanism which hinders a systematic further development of the system.
A further memory cell with an active organic material which exhibits a switchable behaviour is described in Bandyopadhyay et al.: Applied Physics Letters, Vol. 82, 2003, pages 1215-1217 “Large conductance switching and memory effects in organic molecules for data-storage applications”. The manufacture of the active organic layer described therein by Rose Bengal is extremely circuitous and requires several hours of baking in a vacuum, which effectively prevents manufacture.
Memory elements which contain different organic materials are also described by Cölle et al.: Organic Electronics, Vol. 7, 2006. pages 305-312 “Switching and filamentary conduction in non-volatile organic memories”. Various metal/organic/metal structures are investigated here. The memory behaviour is caused by a thin oxide layer on the electrodes and the transport of the electrons via filaments. This work also shows that the reproducibility and reliability of the switching performance, the switch voltage and the memory element are very difficult in themselves and depend on many different uncontrollable. causes. Also the underlying switching mechanisms are ultimately unclear which impedes further optimisation of this memory.
Krieger et. al. in “Synthetic Metals. Vol. 122, 2001, pages 199-202 “Molecular analogue memory cell based on electrical switching and memory in molecular thin films” disclose a test structure comprising an array of 8×8 cells of a size 100 μm×100 μm. Between two metal electrodes is a polymer film (polyphenylacetylene) of thickness 100 to 500 nm mixed with 5-7% NaCl. By slowly increasing the voltage to the electrodes the NaCl is separated into Na+−and CL−−ions. These travel towards the electrodes and cause the resistance to change. This can now be exploited to create a memory cell. This structure has the disadvantage that a strong electrical field must he applied to the electrodes for a long time to trigger the diffusion of the ions, i.e. rapid switching is fairly improbable. Furthermore this structure is a volatile storage cell since the electrical field is switched off the ions travel away from the electrodes through their concentration gradients in the layer and thereby the status cannot be retained.
The cited works all have in common the fact that the switching effects are not clearly defined and for example depend on the forming of metallic filaments, the diffusion of ions or metal atoms, or on substances shifting onto contacts.
The company Thin Film Electronics (www.thinfilm.se) together with the company Xaar has developed an organic Ferro-electrical polymer which can act as a non-volatile memory. This approach exploits the ferro-electrical properties of a polymer which can be expected to be more stable in comparison to the aforementioned effects. Typical for components of polymer electronics is their composition of one or two (seldom several) polymer layers due to the problem when precipitating of polymer heterostructures that already-precipitated layers must not be detached again, however there are only two main groups of polymer solvents, namely hydrophilic and hydrophobic. It is therefore hard to produce complicated layer stacks from polymers.
The memory mechanisms cited above can be linked to form memory modules which are based on different technologies. The simplest is the arrangement in a matrix of crossed metallic conducting contacts, as used for example by Krieger et. al.: Synthetic Metals, Vol. 122, 2001, pages 199-202 “Molecular analogue memory cell based on electrical switching and memory in molecular thin films”. This arrangement can be used to produce high memory densities. It is sufficient with respect to the structuring of the memory cells to structure the contact tracks while the intermediately disposed organic(n) layer(s) can be applied over large surfaces whereby the technologically difficult lateral structuring of organic materials can be avoided.